The fall equinox and the first day of autumn arrives on Monday, September 22, 2025, at 12:19 P.M. MDT in the Northern Hemisphere. The equinox occurs at the same moment worldwide.
During the equinox, the sun crosses what we call the “celestial equator”—an imaginary extension of Earth’s equator line into space. The equinox occurs precisely when the Sun’s center passes through this line. When the Sun crosses the equator from north to south, this marks the autumnal equinox; when it crosses from south to north, this marks the vernal equinox.